Here are some photos of the fabrication of my recumbent cargo bicycle. I selected the front end of a women’s frame and the rear triangle of a suspension frame and began by first connecting them together with a long bottom tube. But first, here is the CAD model and the drawing I made to work from:
Then I began the cargo box construction with the side rails of the lower portion. I mitered some tubing and welded it at 45 degrees, then attached that section to the frame, followed by the rear tube at 45 degrees. Then I made the other half.
Then I put in the vertical tube that connects the top and down tubes of the front half.